Biometric authentication is a technique for identifying a person by using a biometric feature such as a fingerprint, a face, a vein, etc. In biometric authentication, a biometric feature that has been registered in advance is compared (matched) with biometric feature obtained in a scene where confirmation is needed, and a person is identified on the basis of whether or not the biometric feature matches with the registered template.
When vein authentication is conducted on the basis of a vein of a palm by using a terminal such as a mobile phone having a biometric authentication function, the user captures an image of his or her palm in order to obtain vein information. In such a case, the user captures an image with his or her palm a little distant from the vein sensor (camera for example) of the terminal. In such a case, a captured image 11 obtained from the vein sensor sometimes includes a background 13 in addition to a palm 12 as illustrated in FIG. 1. A terminal having a biometric authentication function identifies a region on the palm 12 (palm region) from the captured image 11 in order to achieve authentication accuracy that is needed. Then, the terminal extracts the palm region from the captured image 11 to generate a process image 14 that includes only the palm 12 as illustrated in FIG. 2, and obtains vein information.
As a technique for identifying a region, there is a technique that converts an input image of an RGB format or a YUV format into an HSV format and sets an appropriate threshold (constant) based on distributions of respective color components in a palm region in the HSV space in order to determine whether or not the region is a palm (threshold method). A technique of detecting a flesh color region from an HSV space by using a threshold method is known (see Patent Document 1 for example).
A YUV format is wildly used for an image output by a camera device that is used as a sensor. When a palm region is to be detected in a YUV space, a threshold method sometimes results in erroneous detection of a palm region, deteriorating authentication accuracy.
[Patent Document 1] International Publication Pamphlet No. 2012/173001
[Patent Document 2] Japanese Laid-open Patent Publication No. 2012-14535
[Patent Document 3] Japanese Laid-open Patent Publication No. 2006-260401
[Patent Document 4] Japanese Laid-open Patent Publication No. 2006-31690